Polydienes may be produced by solution polymerization, wherein conjugated diene monomer is polymerized in an inert solvent or diluent. The solvent serves to solubilize the reactants and products, to act as a carrier for the reactants and product, to aid in the transfer of the heat of polymerization, and to help in moderating the polymerization rate. The solvent also allows easier stirring and transferring of the polymerization mixture (also called cement), since the viscosity of the cement is decreased by the presence of the solvent. Nevertheless, the presence of solvent presents a number of difficulties. The solvent must be separated from the polymer and then recycled for reuse or otherwise disposed of as waste. The cost of recovering and recycling the solvent adds greatly to the cost of the polymer being produced, and there is always the risk that the recycled solvent after purification may still retain some impurities that will poison the polymerization catalyst. In addition, some solvents such as aromatic hydrocarbons can raise environmental concerns. Further, the purity of the polymer product may be affected if there are difficulties in removing the solvent.
Polydienes may also be produced by bulk polymerization (also called mass polymerization), wherein conjugated diene monomer is polymerized in the absence or substantial absence of any solvent, and, in effect, the monomer itself acts as a diluent. Since bulk polymerization is essentially solventless, there is less contamination risk, and the product separation is simplified. Bulk polymerization offers a number of economic advantages including lower capital cost for new plant capacity, lower energy cost to operate, and fewer people to operate. The solventless feature also provides environmental advantages, with emissions and waste water pollution being reduced.
Lanthanide-based catalyst systems that comprise a lanthanide compound, an alkylating agent, and a halogen source are known to be useful for producing conjugated diene polymers having high cis-1,4-linkage contents. Nevertheless, when applied to bulk polymerization of conjugated dienes, lanthanide-based catalyst systems generally provide cis-1,4-polydienes having a molecular weight distribution of more than 2.5.
It is known that cis-1,4-polydienes having a narrower molecular weight distribution give lower hysteresis. It is also known that cis-1,4-polydienes having higher cis-1,4-linkage content exhibit the increased ability to undergo strain-induced crystallization and thus give superior physical properties such as higher tensile strength and higher abrasion resistance. Therefore, it is desirable to develop a method for producing cis-1,4-polydienes having a higher cis-1,4-linkage content and a narrower molecular weight distribution.
Preformed lanthanide-based catalysts have been described. These catalysts have been prepared by mixing (a) a conjugated diene monomer, (b) an organic phosphoric acid salt of a rare earth metal, (c) a trialkylaluminum compound or a dialkylaluminum hydride, and (d) an alkylaluminum halide, followed by aging the mixture for a certain period of time prior to bringing the preformed catalyst into contact with the conjugated diene monomer that is to be polymerized. However, the preformed catalysts are less convenient to be employed in a commercial production process because the preparation, aging, and storing of the preformed catalysts requires a separate reaction vessel in addition to the polymerization vessel. In addition, the activity, selectivity, and other performance characteristics of the preformed catalysts can undergo alteration during aging and storage, which causes difficulty in controlling the polymerization process and obtaining desired polymer properties. For these reasons, it is often advantageous to employ a catalyst that is formed in situ.